The invention generally relates to systems and methods for validating the instrument landing system positioning information of an airplane conducting a precision approach to a runway.
An instrument landing system (ILS) is a ground-based instrument approach system that provides precision guidance to an aircraft approaching and landing on a runway, using a combination of radio signals and, in many cases, high-intensity lighting arrays to enable a safe landing during instrument meteorological conditions, such as low ceilings or reduced visibility due to fog, rain, or snow.
Instrument approach procedure charts (or approach plates) are published for each ILS approach, providing pilots with the needed information to fly an ILS approach during instrument flight rules operations, including the radio frequencies used by the ILS components and the minimum visibility requirements prescribed for the specific approach.
An ILS includes ground-based transmitters, located at runways, and airborne receivers. The ILS transmitters transmit signals, received by the receivers on the aircraft, which are utilized to align an aircraft's approach to a runway. Typically, an ILS consists of two portions, a localizer portion and a glide slope portion. The localizer portion is utilized to provide lateral guidance and includes a localizer transmitter located at the far end of the runway. The glide slope portion provides vertical guidance to a runway and includes a glide slope transmitter located at the approach end of the runway. Aircraft guidance is provided by the ILS receivers in the aircraft by performing a modulation depth comparison.
More specifically, a localizer (LOC) antenna array is normally located beyond the departure end of the runway and generally consists of several pairs of directional antennas. Two signals are transmitted at a carrier frequency. One is modulated at 90 Hz; the other at 150 Hz. These modulated signals are transmitted from separate but co-located antennas. Each antenna transmits a narrow beam, one slightly to the left of the runway centerline, the other to the right.
The localizer receiver on the aircraft measures the difference in the depth of modulation (DDM) of the 90 and 150 Hz modulated signals. For the localizer, the depth of modulation for each of the modulating frequencies is 20 percent. The difference between the two signals varies depending on the deviation of the approaching aircraft from the centerline. If there is a predominance of either modulated signal, the aircraft is off the centerline. In the cockpit, the needle on a horizontal situation or course deviation indicator will show that the aircraft needs to fly left or right to correct the error to fly down the center of the runway. If the DDM is zero, the aircraft is on the centerline of the localizer coinciding with the physical runway centerline.
A glide slope (GS) antenna array is sited to one side of the runway touchdown zone. The GS signal is transmitted on a carrier frequency using a technique similar to that of the localizer. The centerline of the glide slope signal is arranged to define a glide slope of approximately 3° above horizontal (ground level). The beam is 1.4° deep; 0.7° below the glideslope centerline and 0.7° above the glideslope centerline.
The localizer and glide slope both provide an angular deviation from a desired flight path. The angular deviation is the angle between the current flight path and the desired flight path.
The pilot controls the aircraft so that the indications on the course deviation indicator remain centered on the display. This ensures the aircraft is following the ILS centerline (i.e., it provides lateral guidance). Vertical guidance, shown on the instrument by the glideslope indicator, aids the pilot in reaching the runway at the proper touchdown point. Many aircraft possess the ability to route signals into the autopilot, allowing the approach to be flown automatically by the autopilot.
The output from the ILS receiver goes both to the display system (head-down display and head-up display, if installed) and can also go to the flight control computer. An aircraft landing procedure can be either “coupled,” where the flight control computer directly flies the aircraft and the flight crew monitor the operation; or “uncoupled” (manual), where the flight crew fly the aircraft using the primary flight display and manually control the aircraft to minimize the deviation from flight path to the runway centerline.
There are three categories of ILS which support similarly named categories of operation.
Category I (CAT I)—A precision instrument approach and landing with a decision height not lower than 200 ft above touchdown zone elevation and with either a visibility not less than 2,625 ft or a runway visual range (RVR) not less than 1,804 ft.
Category II (CAT II)—A precision instrument approach and landing with a decision height lower than 200 ft above touchdown zone elevation but not lower than 100 ft, and a RVR not less than 984 ft for aircraft approach category A, B, C and not less than 1,148 ft for aircraft approach category D.
Category III (CAT III) is subdivided into three sections:
Category III A—A precision instrument approach and landing with: (a) a decision height lower than 100 ft above touchdown zone elevation, or no decision height (alert height); and (b) a RVR not less than 656 ft.
Category III B—A precision instrument approach and landing with: (a) a decision height lower than 50 ft above touchdown zone elevation, or no decision height (alert height); and (b) a RVR in the range 246-656 ft.
Category III C—A precision instrument approach and landing with no decision height and no RVR limitations.
Runway Visual Range (RVR) is a term used in aviation meteorology to define the distance over which a pilot of an aircraft on the centerline of the runway can see the runway surface markings delineating the runway or identifying its centre line.
RVR is used as one of the main criteria for minima on instrument approaches, as in most cases a pilot must obtain visual reference of the runway to land an aircraft. RVRs are transmitted by air traffic controllers to aircraft making approaches to allow pilots to assess whether it is prudent and legal to make an approach.
An ILS is required to shut down upon internal detection of a fault condition. With the increasing categories, ILS equipment is required to shut down faster, since higher categories require shorter response times. For example, a CAT I localizer must shutdown within 6 seconds of detecting a fault, but a CAT III localizer must shut down in less than 2 seconds.
When conducting an ILS approach, the airplane uses radio signals from the ILS system for guidance. The higher-class ILS systems have more protections and monitoring in place to ensure that radio signals are not interfered with and internal errors do not cause the guidance to fall out of required accuracy tolerances. This increases the cost of installing and maintaining the ILS systems and is one reason why there are relatively few CAT II and CAT III runways. ILS approaches currently account for the majority of precision approaches at runways around the world. Of these, only about 8% of all ILS systems support CAT II or CAT III operations.
There is a need for a system and method for increasing the positioning assurance of a Type I ILS system, so that the number of runways that support CAT II operations could be increased significantly.